LEDs are used for a variety of lighting applications (e.g., full-color displays, lamps, traffic lights, holiday lights, etc.), and are increasingly finding additional applications as LED technology improves and the cost of LEDs decreases.
LEDs are becoming increasingly more efficient due to continuous improvements in LED fabrication and LED design. However, a general limitation on LED light emission efficiency is due to total internal reflection of the light generated within the LED. For example, in a gallium-nitride- (GaN)-based LED, n-doped and p-doped GaN layers are supported by a semiconductor substrate (e.g., sapphire) having a surface. The n-doped and p-doped GaN layers sandwich an active layer, and one of the GaN layers has a surface that interfaces with air. Light is generated in the active layer and is emitted equally in all directions. However, GaN has a relatively high refractive index of about 3. As a result, there exists at the GaN-air interface a maximum-incident-angle cone (“exit cone”) within which the light exits the p-GaN-air interface, but outside of which light is reflected back into the GaN structure due to Snell's Law.
To improve LED light emission efficiency, certain LEDs have been fabricated with a roughened substrate surface. The roughened substrate surface scatters the internally reflected light, causing some of the light to fall within the exit cone and exit the LED, thereby improving the light emission efficiency of the LED.
In a manufacturing environment, it is desirable to have a controllable and consistent method of forming the roughened substrate surface so that the LEDs have an identical structure and identical performance. The present method of roughening the substrate surface using abrasion is not a repeatable process and is thus not well suited for high-volume LED manufacturing.